Formation of Massive Primordial Stars in a Reionized Gas

TL;DR

This study uses high-resolution cosmological simulations to demonstrate that primordial gas clouds in reionized regions can form massive stars around 40 solar masses, which significantly impact early universe chemical enrichment and high-redshift phenomena.

Contribution

The paper presents detailed simulations showing the formation of ~40 solar mass primordial stars in reionized gas, highlighting their stability and potential role in early cosmic evolution.

Findings

Primordial gas clouds collapse to form ~40 Msun stars.

The proto-stellar core remains stable against instabilities.

Massive primordial stars influence early universe enrichment.

Abstract

We use cosmological hydrodynamic simulations with unprecedented resolution to study the formation of primordial stars in an ionized gas at high redshifts. Our approach includes all the relevant atomic and molecular physics to follow the thermal evolution of a prestellar gas cloud to very high densities of ~10^{18} cm^{-3}. We locate a star-forming gas cloud within a reionized region in our cosmological simulation. The first run-away collapse is triggered when the gas cloud's mass is ~40 Msun. We show that the cloud core remains stable against chemo-thermal instability and also against gravitational deformation throughout its evolution. Consequently, a single proto-stellar seed is formed, which accretes the surrounding hot gas at the rate ~10^{-3} Msun/year. We carry out proto-stellar evolution calculations using the inferred accretion rate. The resulting mass of the star when it reaches the zero-age main sequence is M_ZAMS ~40 Msun. We argue that, since the obtained M_ZAMS is as large as the mass of the collapsing parent cloud, the final stellar mass should be close to this value. Such massive, rather than exceptionally massive, primordial stars are expected to cause early chemical enrichment of the Universe by exploding as black hole-forming super/hypernovae, and may also be progenitors of high redshift gamma-ray bursts. The elemental abundance patterns of recently discovered hyper metal-poor stars suggest that they might have been born from the interstellar medium that was metal-enriched by supernovae of these massive primordial stars.